The present invention relates to apparatus for making initial tack tests for various adhesives. The apparatus prescribed by the U.S. Army for making such initial tack tests is described in paragraph 4.4.4 of Federal Specification MMM260A, substantially as follows:
"4.4.4 Initial Tack. Separate tests shall be made using class H-4 and L-2 materials at a temperature of 73.degree. .+-. 2.degree. F., and 50 .+-. 5 percent relative humidity. By means of suitable adhesive, a specimen of the barrier materials shall be securely bonded to one end of (each of) two smooth, clean steel cylinders, each having a diameter of 1 .+-. 0.009 inch, weighing 12 ounces .+-. 0.010 ounce, and approximately 3 inches long. The ends of the cylinders shall be clean, and flat squared perpendicular to their lengths. The bonded barrier material applied as above shall be trimmed carefully so that a disc is formed closely conforming to the one inch diameter of the cylinder. With each cylinder in an upright position, a film of the adhesive covered by this specification shall be applied in the thickness recommended by the manufacturer to each barrier material disc and allowed to dry for 30 seconds (except that class 3 -- hot melt adhesives shall be subjected to the next operation within 10 seconds of adhesive film coating discs), the time being determined from the beginning of the adhesive application. A duplicate cylinder (with similar adhered discs) shall be placed on top of the first, so that the cylinders are aligned and the barrier material discs are adjacent, with only the specification adhesive between. The crepe direction of the barrier material discs shall be in the same direction and care taken in placing the top cylinder on the lower cylinder not to apply any pressure other than the weight of the upper cylinder. Specimens having discs of H-4 barrier material shall have an additional 12 ounce load applied on the upper cylinder. Sixty seconds after applying the cylinders together (80 seconds for class 3-hot melt adhesives), each assembly shall be raised by lifting the upper bonded cylinder until the bottom cylinder is free of the supporting surface. A total of 10 assemblies (20 bonded cylinders) using each of the above barrier materials (in disc form) shall be so tested. A minimum of 7 of the 10 specimens of each variety shall be so raised without separation of the adhesive bond between the discs in order to comply with the requirement for initial tack."
A major criticism of paragraph 4.4.4 is that the test results are more a measure of skill of the technician in specimen preparation than the initial tack of the adhesive being measured. The diameter of the test discs result in large variations in the effective bonding area. There is often a great deal of scatter on repeated tests with the same material.
An object of the present invention is to provide an entirely different, and improved, apparatus and method for making initial tack tests for adhesive bonds.
In accordance with the invention, the improved apparatus comprises a U-shaped bracket adapted to be supported in a horizontal plane, e.g. on a table; a tiltable elongated panel pivotally supported within the U for tilting from a horizontal to a vertical position; a first adherend affixed to the panel; at least one elongated second adherend adhesively bonded at least at one end to a portion of the first adherend that is up in the vertical position; and a weight affixed to the second adherend to apply a constant gravity load to the adhesive bond in the vertical position. Preferably, the first adherend is a sheet, covering the panel, to the upper edge of which the upper ends of several strip adherends are adhesively bonded, with a seperate weight on each strip adherend. The strip adherends may be either fold-bonded to the sheet adherend for a peel test, or flat-bonded for a shear test. Each adherend is preferably made of two layers of Kraft paper bonded by a thin layer of asphalt. An elongated block may be attached to the bracket legs to stop the panel in its vertical position. The invention provides an apparatus that can be used with great reliability by technicians with little or no training.